I think people like seeing themselves as centerists as they can think of themself as some pragmatist who takes the best ideas from each side, which infers that the people on either side of them or incapable of coherent thought and just blindly follow thier dogma...

Of course there are those on each side who do blindly follow thier sides view, and then of course there are those that are actually centerist...

But the big clamour to be seen as a centerist is partially to make yourself look pragmatic partially so you can be more convincing in arguments to both sides...

The last part is true for me... if someone i agree with often takes a position contary to mine it makes me question myself more... if its someone i usually disagree with it would reinforce my position if anything (though i don't intentionally the effect could be there and i am blind to it...)